1. Field of Invention
The invention relates generally to optical sources and in particular to compensation of thermal characteristics of a frequency stabilized optical source.
2. Description of Related Art
Recently, the channel density of commercial Wavelength Division Multiplexing (WDM) systems has increased dramatically, resulting in narrower frequency spacing between channels. Narrow channel spacing, on the order 25 GHz or 12.5 GHz, is often very sensitive to crosstalk caused by frequency drifts in which a channel interferes with an adjacent channel. To address this frequency drift and facilitate wavelength locking, optical device suppliers have integrated wavelength monitors with the optical source.
FIG. 1A is a block diagram of an optical device 10 with a frequency controller 60 (also referred to as a locker). The optical device 10 includes an optical source 20 with an integrated frequency reference element 30. The optical source 20 may include, but is not limited to, a distributed feedback (DFB) laser, other lasers, and the like, as well as combinations including the foregoing. The frequency reference element 30 is a component that translates the frequency of the output of the optical source 20 to an amplitude. An exemplary frequency reference element 30 is a Fabry-Perot etalon filter.
Light emitted from the rear facet of the optical source 20 is transmitted via a beam splitter to a first detector 40 and a frequency reference element 30 and thereafter to a second detector 50. The first detector 40 and second detector 50 may include but not limited to photo detectors, photodiodes, phototransistors, and the like, as well as combinations including the foregoing. The first detector 40 produces a current indicative of the total optical output power denoted Ipf. The second detector 50 produces a current indicative of a wavelength dependent optical power denoted Ixcex. The optical power, as measured by the first detector 40 and second detector 50, is transmitted as currents Ipf and Ixcexrespectively, to controller 60.
FIG. 1B is a side view depicting one arrangement of components in the optical device 10. As shown in FIG. 1B, the optical source 20 and the frequency reference element 30 may be positioned on the thermal electrical cooler (xe2x80x9cTECxe2x80x9d) 72. As described in further detail, the frequency reference element 30 experiences a temperature gradient due to a difference between the temperature of TEC 72 and the case 11 (or alternatively called a housing) of the optical device 10. This temperature gradient causes the output frequency of the optical source 20 to vary.
The optical output from frequency reference element 30 varies with wavelength so that the current Ixcexis indicative of the wavelength output by optical source 20. FIG. 2 depicts an exemplary discriminator curve when an etalon filter is used for frequency reference element 30. The discriminator curve illustrates that the ratio of Ixcexto Ipf is indicative of the output frequency of the optical source 20. The frequency processing module 62 executed by controller 60 translates currents Ipf and Ixcexinto an error signal that is used by a temperature compensator 70. The temperature compensator 70 adjusts the temperature of the optical source 20 to control the output frequency of the optical source 20.
The temperature compensator 70 includes, but is not limited to, a thermoelectric cooler (TEC) 72, temperature sensor 74 and temperature driver module 64. The temperature driver module 64 is preferably, but not necessarily, integrated with controller 60 to control temperature of the optical source 20. The error signal is received by the temperature driver module 64 which adjusts the temperature of the optical source 20 to reduce the error signal.
As described above, the existing wavelength-locking scheme is primarily composed of a feedback loop where the ratio (Ixcex/Ipf) is monitored. Referring to FIG. 3, the desired frequency is established with a particular reference point (Ixcex/Ipf)REF 102 on the discriminator curve corresponding to a selected magnitude of the ratio (Ixcex/Ipf) and resulting in the desired frequency fREF. The feedback functionality implemented in frequency processing module 62 and temperature driver module 64 then adjusts the optical source 20 parameters to ensure that the ratio (Ixcex/Ipf) is maintained at the reference point, (Ixcex/Ipf)REF 102. The optical source parameter that is adjusted can be the drive current, the temperature, or both. In the implementation depicted in FIG. 1 a temperature driver module 64 adjusts the temperature of the optical source 20 to maintain the desired frequency fREF.
FIG. 4B depicts conventional wavelength locker processing with which the operating frequency is detected at step 190. The operating frequency is compared to a reference frequency at step 192 and operating parameters of the optical source 20 are adjusted at step 194.
A drawback to the existing systems is that the characteristics of the frequency reference element 30 change with temperature. Since the frequency reference element 30 may be distanced from the optical source 20, monitoring the temperature through temperature sensor 74 may not accurately reflect the temperature of the frequency reference element 30. As noted above, the frequency reference element 30 may experience a temperature gradient due to a temperature differential between the TEC 72 and the case 11 (or housing) of optical device 10. Variations in the temperature of the frequency reference element 30 shifts the discriminator curve. Thus, locking the frequency based on the same reference point (Ixcex/IPf)REF 104 on the shifted discriminator curve will shift the locked frequency value to fSHIFT. As a result of this temperature dependence for the frequency reference element 30, the operational frequency of a frequency-locked optical source 20 drifts as the case temperature of the optical device 10 is changed. This drift is depicted in FIG. 4A. Such drift in the output frequency of the optical source 20 can result in deleterious effects such as crosstalk between channels.
Therefore, there is a need for a mechanism to reduce the temperature effects on the frequency of the optical device 10.
An embodiment of the invention is a controller for use with an optical device having an optical source and a frequency reference element. The controller includes a frequency processing module coupled to the optical device. The frequency processing module generates an error signal indicative of a deviation between the output frequency of the optical source and a reference frequency corresponding to a reference point. A driver module communicates with the optical device and the frequency processing module. The driver module adjusts a parameter of the optical source in response to the error signal. An offset processing module is coupled to the frequency processing module. The offset processing module derives an offset signal based on an estimate of a temperature of the frequency reference element. The offset processing module provides the offset signal to the frequency processing module which updates the reference point in response to the offset.